Osthoff's Law is the tendency for Proto-Indo-European long vowels to shorten before sonorants (y w r l m n) in closed syllables. This technically applies to its occurence in Greek only, where it is regular.

For example, PIE *dyēws becomes Greek Ζευς, and not *Ζηυς (which would equal Sanskrit dyāus).

The Greek long diphthongs that are attested arose from different sources (such as contraction).

Named after Hermann Osthoff.